[datatable-help] Conditional replacement using a single line

[Eduard Antonyan]

DT[, value := c(2, rep(0, .N-1)), by = id]

On Tue, Aug 25, 2015 at 11:17 AM, Frank S. <f_j_rod at hotmail.com> wrote:

> Hello everyone,
> I ask a question directly linked with other question which I was answered
> in this list a year ago. Let the data table be:
> DT <- data.table(obs=1:7, id=c(1,1,1,4,4,4,4), time=c(3,4,7,5,8,10,15))
>
> Now I add a new column, called "value", which contains all zeros except
> the first observation within each "id" group, which is equal to 2.
> One possible solution I was given is the following two code lines:
> > DT[ , value:=0]
> > DT[!duplicated(id), value:=2]
>
> But I wonder if it is possible to do the same ON A SINGLE CODE LINE. I
> have tried different options but they doesn't work:
> 1) DT[ , value:= c(0, 2), by=id]
> 2) DT[ , value:= ifelse( DT[!duplicated(id)]==T, 2, 0), by=id]
>
> Many thanks!
>
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